Silver non-corrosive lubricants



corrosive to silver and like metals.

United States Patent SILVER NON-CORROSIVE LUBRICANTS James 0. Clayton and Warren Lowe, Berkeley, Calif., assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing. Filed Apr. 9, 1956, Ser. No. 576,797

2 Claims. (Cl. 252-32.7)

This invention pertains to lubricating oil compositions which are resistant to oxidation and which are also non- In particular, this invention pertains to lubricating oil compositions containing polyvalent metal salts of diesters of dithiophosphoric acids and alkaline earth metal glycoxides.

This application is a continuation-in-part of Clayton- Lowe application Serial No. 232,480, filed June 19, 1951, and now abandoned.

Generally, oils of lubricating viscosity are subjected to extreme conditions during their use as lubricants. Under present-day services, lubricating oil compositions are subjected to higher temperatures and greater conditions of wear than those which existed only a relatively few years ago. In attempting to improve upon the lubricating oils, that is, to improve their stability to oxidation and the temperature-viscosity relationships, for example, the petroleum fractions have been subjected to greater degrees of refinement. Although this higher refinement did accomplish the purpose of improving the viscositytemperature relationships of the oil, the oil become more susceptible to oxidation. Similarly, numerous synthetic oils have been prepared in order to obtain an oil having improved viscosity-temperature relationships. Even synthetic oils have not been sufiiciently stable to oxidation to permit their use alone as such.

Thus, in order to increase the oxidation stability of lubricating oils it is essential to incorporate oxidation inhibitors therein. However, the oxidation inhibitors which are more frequently used (e.g., sulfurized olefins, sulfurized diparafiin sulfides, dialkyl phenyl disulfides, polyvalent metal salts of esters of thio-pho-sphoric acids, etc.) are corrosive to silver and like metals; that is, silver bearings are readily attacked by these oxidation inhibitors. When using polyvalent metal salts of esters of thio-phosphoric acids (e.g., zinc dicetylphenyl dithiophosphate) as the oxidation inhibitor, this corrosion to silver can be inhibited in accordance with this invention as described hereinbelow. However, this is not the case when other oxidation inhibitors noted hereinabove are used, in which cases the corrosivity to silver actually increases.

It is a primary object of this invention to prepare lubricating oil compositions which are stable to oxidation and which at the same time resist corrosion to silver and like metals.

According to this invention, it has been discovered that the silver corrosivity of lubricating oil compositions containing zinc salts of esters of dithiophosphoric acids can be inhibited by incorporating therein alkaline earth metal glycoxides. The solution (or colloidal dispersion) of the alkaline earth metal glycoxides in the lubricating oil compositions is stabilized by the presence of polyvalent metal sulfonates.

The lubricating oil compositions of this invention comprise a major proportion of an oil of lubricating viscosity and minor amounts of zinc salts of esters of dithiophosphoric acids and sulfonate-stabilized dispersions (or solutions) of alkaline earth metal glycoxides.

Thus, calcium glycoxides inhibit the corrosivity to silver and like metals caused by the use of zinc salts of esters of dithiophosphoric acids as oxidation inhibitors in lubricating oil compositions.

tained from a lubricating oil composition containing 0.6 millimol percent of a zinc salt of an ester of thiophos: phoric acid. However, when 5.7% by weight of a calcium glycoxide dispersion (defined hereinbelow) was incorporated in the oil with the zinc salt, the silver strip corrosion loss was remarkably reduced to 11.8 mg.

On the other hand, when 5.7% of this same calcium glycoxide dispersion was added to a lubricating oil containing 1% by weight of a dialkylphenyl disulfide as an oxidation inhibitor, the silver strip loss increased from 136.5 mg. to 197.7 mg. Similarly, the addition of 5.7% of this same calcium glycoxide dispersion to a lubricating oil containing 0.5% by weight of sulfurized olefins as oxidation inhibitors the silver corrosivity increased from 137.2 mg. to 163.5 mg. Furthermore, the silver strip loss of a lubricating oil containing 1% by weight of sulfurized diparafiin sulfide was 109.2 mg, while the silver strip loss of the same oil composition to which had been added 5.7 by weight of the calcium glycoxide dispersion was 141.5 mg.

Thus, it is readily seen that the use of alkaline earth metal glycoxides according to this invention to inhibit silver corrosivity caused by polyvalent metal salts of esters of dithiophosphoric acids is unique.

In the formation of the lubricating oil compositions herein containing alkaline earth metal glycoxides, the oxide or hydroxide of alkaline earth metals is used as the initial reactant. The lubricating oil compositions of this invention are prepared by dissolving an alkaline earth metal oxide or hydroxide (preferably calcium oxide or hydroxide) in a dihydric alcohol (e.g., ethylene glycol). The reaction is described in Industrial and Engineering Chemistry, vol. 8, No. 9, page 451. This solution is then incorporated into a lubricating oil con taining an oil-soluble alkaline earth metal dispersant (e.g., an alkaline earth metal sulfonate), after which the mixture is heated to remove the free dihydric alcohol, and filtered, forming a clear, filterable lubricating oil composition. acid may be added to the lubricating oil before the filtra tion step or after the filtration step. In order to facilitate filtration, a filtering agent may be added before filtration.

Thus, lubricating oil compositions containing alkaline earth metal glycoxides are formed by heating a mixture of a lubricating oil, a dispersant, an alkaline earth metal oxide (or hydroxide), a dihydric alcohol, and a filtering agent to a temperature of about F. to about 400 F. This heating is for a period of time sufiicient to promote the reaction between the alkaline earth metal oxides or hydroxides and the dihydric alcohol; to disperse the resulting alkaline earth metal glycoxide in the lubricating oil compositions; and to remove substantially all of the remaining apparently unreacted dihydric alcohol; after which the mixture is filtered to remove any apparently unreacted alkaline earth metal oxide or hydroxide.

Other methods which can be used in the preparationof the lubricating oil compositions of this invention include" the following: A solution of calcium oxide (or hydroxide) in a benzene-dihydric alcohol blend is incorporated into a lubricating oil containing an alkaline earth' metal dispersant. The whole mixture is heated until all of the benzene and substantially all of the dihydric alcohol has been removed, after which the mixture is filtered.

In another method, a mixture of a dihydric alcohol, an alkaline earth metal sulfonate, and a calciumsaltof a low molecular weight alcohol (e.g., calcium ethylate) Patented Mar. 14, 1961 For example, a silver strip corrosion loss (defined hereinbelow) of 120.0 mg. was ob- The zinc salts of esters of dithiophosphoric is incorporated into a lubricating oil. The whole mixture is slowly heated until the low molecular weight alcohol and substantially all of the dihyd'ric alcohol are removed, after which the whole mixture is filtered.

The dihydric alcohols used in obtaining the lubricating oil composition of this invention are glycols containing less than carbon atoms; in particular, these glycols include vicinal-alkane diols having less than 5 carbon atoms. Suitable dihydric alcohols include, for example, ethylene glycol, propane diol-1,2; butane di0l-2,3; butane diol-l,2; etc. Ethylene glycol is preferred, because thereby, greater amounts of metal base are incorporated in the lubricating oil compositions.

The inorganic substances reacting with the dihydric alcohols to form the glycoxides are the oxides or hydroxides of the alkaline earth metals; more particularly, the oxides or hydroxides of calcium.

The zinc salts of esters of dithiophosphoric acids are represented by the following formula:

wherein R and R may be alkyl, aryl, alkaryl, aralkyl, or cyclic nonbenzenoid hydrocarbon radicals containing a total of from 7 to 50 carbon atoms. It is preferred that the R radical contain from 1 to 25 carbon atoms, and that the R radical contain from 6 to 25 carbon atoms. R and R may or may not be identical.

When R and R are identical and consist of the lower molecular weight hydrocarbon radicals (e.g., butyl radicals), the zinc salt normally does not have sutficient oil solubility to permit its use in lubricating oil compositions. However, when the R and R are different but still of low molecular weight (e.g., when R is a butyl radical and R is a hexyl radical), it is possible to prepare zinc salts of mixed esters of dithiophosphoric acids which are sufliciently oil soluble to inhibit the oxidation of the lubricating oil composition. The advantages of the use of low molecular weight R and R groups also include the decreased cost over that of the high molecular weight organo radicals. Thus, in the practice of this invention, it is preferred to use zinc salts of mixed esters of dithiophosphoric acids wherein R is derived from an alcohol containing less than 4 carbon atoms; for example, methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tertiary butyl, etc., and wherein R is derived from alcohols containing from 6 to 18 carbon atoms, including hexyl, methylisobutylcarbinol, methylisopropylcarbinol, heptyl, isoheptyl, 2-ethylamyl, octyl, isooctyl, 3-ethylhexyl, 2-propylamyl, decyl, undecyl, dodecyl, hexadecyl, octadecyl, etc.

Examples of R and R when these are the same include the following radicals: octyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, hexylphenyl, decylphenyl, dodecylphenyl, hexadecylphenyl, octadecylphenyl, etc.

The esters of dithiophosphoric acids used in the preparation of the zinc salts of this invention include butyl hexyl dithiophosphoric acid, methyl hexyl dithiophosphoric acid, ethyl hexyl dithiophosphoric acid, butyl methylisobutylcarbinol dithiophosphoric acid, butyl heptyl dithiophosphoric acid, butyl decyl dithiophosphoric acid, butyl isoheptyl dithiophosphoric acid, butyl octadecyl dithiophosphoric acid, dioctyl dithiophosphoric acid, diheptyl dithiophosphoric acid, dihexadecyl dithiophosphoric acid, dioctadecyl dithiophosphoric acid, didectylphenyl dithiophosphoric acid, etc.

The filtering agents used in the filtration of the lubrieating oil composition consist of the combination of low molecular weight, non-hydroxy, monobasic carboxylic acids and alpha-hydroxy acids.

The low molecular weight non-hyd'roxy monobasic carboxylic acids include formic acid and acetic acid. The alkaline earth metal salts (particularly the calcium salts) of these low molecular weight monobasic carboxylic acids may also be used in place of the acids per se, or mixtures thereof.

Alpha-hydroxy acids which are used according to this process include glycolic acid and lactic acid. Instead of the alpha-hydroxy acids themselves, alkaline earth metal salts (particularly the calcium salts) of the alphahydroxy acids can be used, or mixtures thereof.

Polyvalent metal sulfonates used herein include oilsoluble alkaline earth metal salts of sulfonic acids containing hydrocarbon radicals derived from alkanes exemplified by dodecane, hexadecane, eicosane, triacontane, etc.; radicals derived from petroleum hydrocarbons, such as white oil, wax, or olefin polymers (e.g., polypropylene and polybutylene, etc.). The sulfonic acids used in preparing the sulfonates of this invention also include the oil-soluble sulfonic acids obtained from pctroleum, such as mahogany acids, and the synthetic sulfonic acid prepared by various methods of synthesis (e.g., sulfonic acid prepared by reacting a chlorinated white oil with benzene, using hydrofluoric acid as the catalyst, then treating the resulting white oil alkylated benzene with chlorosulfonic acid or fuming sulfuric acid to form a white oil benzene sulfonic acid).

Suitable oils of lubricating viscosity in which the polyvalent metal salts of mixed esters of dithiophosphoric acids and the calcium glycoxides may be incorporated include a wide variety of lubricating oils such as naphthenic base, parafiin base, and mixed base mineral oils, other hydrocarbon lubricants, e.g., lubricating oils derived from coal products, and synthetic oils, e.g., alkylene polymers (such as polymers of propylene, butylene, etc., and mixtures thereof), alkylene oxide type polymers, dicarboxylic acid esters and liquid esters of acids of phosphorus. Synthetic oils of the alkylene oxide type polymers which may be used include those exemplified by the alkylene oxide polymers (e.g., propylene oxide polymers) and derivatives, including alkylene oxide polymers prepared by polymerizing alkylene oxide, e.g., propylene oxide, in the presence of water or alcohols, e.g., ethyl alcohol, and esters of alkylene oxide type polymers (e.g., acetylated propylene oxide polymers) prepared by acetylating propylene oxide polymers containing hydroxyl groups.

Synthetic oils of dicarboxylic acid ester type include those which are prepared by esterfying such dicarboxylic acids as adipic acid, azelaic acid, suberic acid, sebacic acid, alkenyl succinic acid, fumaric acid, maleic acid, etc., with alcohols such as butyl alcohol, hexyl alcohol, Z-ethylhexyl alcohol, dodecyl alcohol, etc. Examples of dicarboxylic acid ester synthetic oils include dibutyl adipate, dihexyl adipate, di-Z-ethylhexyl sebacate and the polyester of 1,3-butylene glycol and adipic acid.

Synthetic oils of the type of liquid esters of acids of phosphorus include the esters of phosphoric acid, e.g., tricresyl phosphate; the esters of phosphonic acids, e.g., diethyl ester of decane phosphonic acid (or other esters as obtained by reacting alkyl phosphonyl chlorides with hydroxy-containing compounds such as phenols and aliphadtic alcohols), and with olefin oxides such as propylene 0x1 e.

Zinc salts of esters of dithiophosphoric acids are excellent lubricating oil additives for inhibiting corrosion of copper-lead and cadmium-silver bearings, which corrosion results from oxidation. However, such corrosion is not the same type of corrosion brought bought by the use of zinc salts herein in the lubrication of silver bearings, wherein the silver bearings are considered to be substantially pure silver.

The zinc salts of esters of dithiophosphoric acids are normally used in lubricating oil compositions in amounts sufficient to inhibit oxidation of the base oils. Although it is preferred to use from 0.05% to 2.0% by weight based on the final oil composition, lubricating oil con- 'ao ratsi greater amount of the basic materials dispersed in the lubricating oil than the use of a higher molecular weight dihydric alcohol (e.g., propylene glycol), when both are used in the same amounts by weight. It is preferred to select a dihydric alcoholiu which the dispersants and the alkaline earth metal oxides or hydroxides havethe greatest solubility.

is beneficial to use certain ratios by .weight of the dihydric alcohol to the alkaline earthmetal oxide or hydroxide. These ratios may vary from about 50:1 to about 2:1, 30:1 to about 10:1 being preferred.

The amount of alkaline earth metal glycoxides which are dispersible in the lubricating oil compositions herein is dependent on the amount of dispersant which is present. Normally, one part by weight of a polyvalent metal sulfonate can stably disperse as much as 0.2 part or more by weight of the glycoxide. For example, one part by weight of an alkaline earth metal mahogany petroleum sulfonate can stably disperse 0.2 part by weight of an In the formation of solutions of. alkaline earth metal oxides or hydroxides in glycols, it.

alkaline earth metal glycoxide. Expressed otherwise, one

mol of an alkaline earth metal mahogany petroleum sulfonate can stably disperse at least 1.75 mols of an alkaline earth metal glycoxide. Thus, the alkaline earth metal glycoxides and dispersants are present in the lubricating oil compositions in such amounts that theglycoxide/dispersant mol ratios have values of at least 1.75, and as high as 4.0; preferably, from about 2.0 to 3.0.

In terms of the alkaline earth metal oxides and hydroxides which are initially incorporated into the lubricating oil compositions, the amounts of these oxides and hydroxides can vary from about 0.02% to about 7.0%,.

by weight.

' The sulfonates can be used in amounts of 0.1% to 10% by weight of the total composition. However, because lubricating oil compositions containing from 0.3% to 2% of the dispersants markedly increase the over-all rating of an engine, it is preferred to use these latter. amounts.

Although numerous methods are available for the preparation of the zinc salts of this invention, it is preferred to prepare the zinc salts from esters of dithiophosphoric acids prepared by reacting phosphorus pentasulfide with an alcohol at temperatures ranging from about 120 F. to 200 F. Where the dithiophosphoric acid esters contain two different organo radicals, the

a mixed esters of dithiophosphoric acids are prepared by "reacting phosphorus pentasulfide with a blend of two diiferent alcohols. For example, in the preparation of mixed esters of dithiophosphoric acids wherein methyl alcohol and hexyl alcohol are the two alcohols, 4 mols of the desired alcohols (e.g., 3 mols of methyl alcohol and 1 mol of hexyl acohol, wherein the mol ratio of methyl alcohol to hexyl alcohol is 3:1) are reacted with 1 mol of P 8 at temperatures ranging from about 120 F. to 180 F. for a time sufiicient to complete the formation of methyl hexyl dithiophosphoric acid. In the preparation of the Zinc salts of the mixed dialkyl dithiophosphoric acids, the mixed dialkyl dithiophosphoric acids are reacted with zinc oxide at temperatures ranging from 70 F. to 170 F.

The following examples illustrate methods of preparing zinc salts of esters of dithiophosphoric acids. Where the organo radicals of the dithiophosphoric acid esters are to be the same, only one alcohol is used in the rea t n,-

EXAMPLE 1 Zinc salt of reaction product of isopropyl alcohol methyl isobutylcarbinol and P S;

A mixture of 252 parts by weight (67 mol percent) reaction vessel. by weight of P 8 i 650 grams of the acid intermediate thus obtained were added'to 130 parts by weight of a petroleum oil having a viscosity of 300 SSU at F. (used as a diluent), then' 122 parts by weight, of zinc oxide were gradually added at F. The whole mixture was then agitated for 3 hours at 130 F., after which the water of neu-' tralization was removed with calcium sulfate. The oil solution of the zinc salt had a pH of 6.7 (after filtration) and contained 9.96% zinc, 9.43% 19.42% sulfur.

EXAMPLE 2 Zinc salt of reaction product of butyl alcohol, methyl isobutylcarbinol, and P 8 A mixture of 237 parts by weight (77 mol percent) of secondary butyl alcohol, 98 parts by weight (23 mol percent) of methyl isobutylcarbinol, and 222 parts by weight of P 8 was charged to a reaction vessel and agitated at for a period of 2 hours. The reaction mixture was cooled and filtered to remove a small amount of unreacted P 5 carbinol dithiophosphoric acid mixture was a dark redgreen liquid having a neutralization number of 193 (mgs. KOH/gram), a viscosity of 35.7 'SSU at 100 5R, a specific gravity of 1.04 (60/60), and contained 24.0% sulfur and 11.9% phosphorus.

To the above mixture was added 87 parts by weight of zinc oxide, after which the whole mixture was heated with agitation at 130 F. for 4 hours until a pH of 6.7 was reached. After the water of neutralization had been re-- moved, the oil solution contained 7.6% zinc, 7.2% phosphorus, and 15.0% sulfur.

In addition to the use of calcium sulfate as a method of removing the water of neutralization in the propara-' tion of the zinc salts, the water of neutralization can be removed by blowing a stream of air through the re- I action mixture during and after the neutralization step.

During transportation and storage, lubricating oil compositions often come into contact with water. through hydration or hydrolysis, this water can reduce the effectiveness of the zinc salts of mixed esters of di-' thiophosphoric acids for the purpose of this invention.

In order to inhibit the water sensitivity of the zinc salts use of a dihydric alcohol in the mixture during the dispersion period (which dihydric alcohol is distilled off from the mixture after the oxides and hydroxides have been dispersed). Preferably the dispersion is obtained or formed by the use of ethylene glycol according to the method set forth in the Lindstrom-Woodruff Patent 2,676,925, filed December 30, 1950. In the preparation 1 of a dispersion, calcium oxide or hydroxide is added to a.

lubricating oil composition containing a dihydric alcohol.

A sulfonate is added to the mixture and the whole mix;

ture is heated to a temperature sufficient to remove the dihydric alcohol. The colloidal dispersion of calcium oxide or hydroxide in oil remains as such, stabilized by 76 the sulfonate.

phosphorus, and

The resulting butyl methyl isobutyl- Possibly I However, the poly- Lubricating oil compositions containing calcium glycoxides are prepared in accordance with this invention by heating a mixture of a dispersant, an inorganic substance selected from the group consisting of alkaline earth metal oxide and/or hydroxide (e.g., calcium oxide and hydroxide), a dihydric alcohol, and a filtering agent in the lubricating oil to the temperature of about 175 F. to about 400 F. (250" F. to 350 F. being preferred) for a period of time sufiicient to promote the' reaction between the alkaline earth metal oxide or hydroxide and the dihydric alcohol, to disperse the glycoxide in the lubricating oil composition, and to remove substantially all of the remaining apparently unreacted dihydric alcohol, after which the mixture is filtered to remove any apparently unreacted alkaline earth metal oxide or bydroxide.

Table I below presents data concerning the effectiveness of the zinc salts of esters of dithiophosphoric acid of this invention in reducing corrosion of metal parts due to oxidation of lubricating oils during operation of internal combustion engines. To obtain these data, a stock Chevrolet engine was used. Instead of the normal Babbitt bearings of the Chevrolet engine, two of the connecting rods were modified to accommodate copper-lead bearing inserts which are weighed before being assembled in the engine. The Chevrolet was operated at 3150 r.p.m., the engine jacket temperature was maintained at 200 F., the crankcase oil temperature was maintained at 280 F., and the engine was operated for a period of 36 hours before being disassembled and the bearing inserts weighed. Before weighing, the bearing inserts were carefully washed with solvents to remove the oil therefrom.

n This reference oil was a California solvent-refined parafiinie base SAE 30 oil containing 0.80% by weight of a calcium petroleum snlfonate and 0.97% of a sulfurized alkyl substituted calcium phcnate.

b Weight loss after 20 hours.

s This reference oil was a Mid-Continent SAE 30 oil.

Tests were also run to determine the effectiveness of the zinc salts of esters of dithiophosphoric acid of this invention for increasing the inhibition period of lubricating oils. A medicinal white oil was the base oil. The inhibition period (the time in hours before 100 grams of oil sample absorbs 1000 cc. of oxygen at 340 F.) of medicinal white oil was zero hours. The inhibition period of the same base oil containing 0.19% of the zinc salt of Example 2 above was 4.2 hours.

In order to evaluate the effectiveness of the alkaline earth metal glycoxides of this invention for inhibiting corrosion to silver, a silver strip corrosion test was made on numerous compositions. This silver strip corrosion test was performed as follows:

A silver metal strip having the dimensions of 2 /2 x /4" x ,4, was first cleaned with a wire brush until the strip was highly polished. The strip was weighed and the weight recorded. This highly polished silver strip was then placed in a 600-milliliter beaker in such a manner that the strip was completely immersed with 300 grams of the oil being tested. The oil was stirred at a temperature of 300 F. for 20 hours, at which time the silver strip was removed and cleaned, first with chloroform,

8 then with petroleum ether. The appearance of the strip was noted. Those strips which had been severely attacked were quite black. The tested silver strip was washed in 15% aqueous solution of potassium cyanide for about 5 minutes to remove the sulfide film adhering to the strip. After the strip had been washed with potassium cyanide and dried, it was weighed. The difference in weight of the original strip and the strip after the potassium cyanide wash was noted and recorded as the weight loss due to corrosion by the zinc metal dialkyl dithiocarbamate.

The following Table II presents data to show the effectiveness of calcium glycoxides in inhibiting the corrosion of silver by zinc salts of esters of dithiophosphoric acids. The reference oil was a California solvent-refined SAE 30 base oil containing 0.6 millimole percent of the zinc salt of the mixed diester dithiophosphoric acids of Example 2 above. The glycoxide dispersion used in the examples was a dispersion in oil of 35 millimole percent calcium glycoxide stabilized with 17.5 millimole percent calcium petroleum sulfonate. The calcium glycoxide was prepared from calcium hydroxide and ethylene glycol.

TABLE II Test Silver Strip N 0. Oil Composition Corrosion Loss (Mgs) Reference Oil 194. 4 Reference 0il+5.7% Glycoxide Dispersiou".. 0.8 Reference Oi1+2.85% Glycoxide Dispersion"- 0. 9 Reference Oil+l.43% Glycoxide Dispersion". 0.4 Reference 0ll+0.67% Glycoxide Dispersinn" 1.0 Reference 0il+0.33% "Glyeoridc Dispersion. l. 7 Reference Oil-+0.15% "Glyeoxide Dispersion" 14. 7

A glycoxide is formed according to the following reaction when a metal oxide is reacted with a dihydric alcohol (e.g., ethylene glycol):

H2CCHz+MO H2C-CHz+I-ia0 H H o-ar-o wherein M is a metal.

The following examples show the formation of glycoxides in the lubricating oil composition herein.

EXAMPLE 3 3.8 mol grams of anhydrous BaO were added to 98.64 grams of anhydrous ethylene glycol. The amount of water resulting therefrom was determined by the Karl Fischer water determination method. The mixture contained 0.46% water (theoretical=0.44%).

EXAMPLE 4 A mixture of 65.0 grams of BaO, 330 grams of ethylene gylcol, and 200 grams of benzene was agitated in a reaction flask. The mixture was heated at reflux temperature of benzene, during which time the water was re moved by azeotropic distillation. 9.5 cc. of a waterethylene glycol mixture were collected in water displacement trap. The refractive index of this water-glycol mixture was 1.3486, which showed that the mixture contained 84% water and 16% ethylene glycol. Therefore, the amount of water resulting from the reaction of the barium oxide with the ethylene glycol was 7.95 grams (theoretical=7.65 grams).

Other agents which may be incorporated into the lubricating oil composition of this invention include oiliness agents, blooming agents, viscosity index improvers, pour point depressants, peptizing agents, thickening agents for greases, etc.

We claim:

1. A hydrocarbon lubricating oil composition consisting essentially of a major portion of an oil of lubricating viscosity, a zinc salt of a diester of dithiophosphoric acid containing a total of from 7 to 50 carbon atoms in the ester radicals, said zinc salt being present in said composition in an amount sutficient to inhibit oxidation of said lubricating oil and to render said composition corrosive to substantially pure silver; from about 0.1% to about 10%, by weight, of oil-soluble, alkaline earth metal sulfonate and an alkaline earth metal glycoxide, said glycoxide being sufiicient in amount to inhibit corrosion of substantially pure silver by said composition, said zinc salt being the only essential sulfur-containing oxidation inhibitor present in said composition.

2. A hydrocarbon lubricating oil composition consist ing essentially of a major proportion of an oil of lubricating viscosity, a zinc salt of mixed diesters of dithiophosphoric acid containing from 1 to 25 carbon atoms in one ester radical'and from 6 to 25 carbon atoms in the other ester radical, said zinc salt being present in said composition in an amount sufi'icient to inhibit oxidation 10 of said lubricating oil and to normally render said composition corrosive to substantially pure silver; from about 0.1% to about 10%, by weight, of an oil-soluble alkaline earth metal sulfonate, and an alkaline earth metal glycoxide, said glycoxide being sufficient in an amount to inhibit corrosion of substantially pure silver by said composition, said zinc salt being the only essential sulfurcontaining oxidation inhibitor present in said composition.

References Cited in the file of this patent UNITED STATES PATENTS 2,079,051 Sullivan et a1 May 4, 1937 2,417,876 Lewis et al. Mar. 25, 1947 2,676,925 Lindstrom et al. Apr. 27, 1954 

1. A HYDROCARBON LUBRICATING OIL COMPOSITION CONSISTING ESSENTIALLY OF A MAJOR PORTION OF AN OIL OF LUBRICATING VISCOSITY, A ZINC SALT OF A DIESTER OF DITHIOPHOSPHORIC ACID CONTAINING A TOTAL OF FROM 7 TO 50 CARBON ATOMS IN THE ESTER RADICALS, SAID ZINC SALT BEING PRESENT IN SAID COMPOSITION IN AN AMOUNT SUFFICIENT TO INHIBIT OXIDATION OF SAID LUBRICATING OIL AND TO RENDER SAID COMPOSITION CORROSIVE TO SUBSTANTIALLY PURE SILVER; FROM ABOUT 0.1% TO ABOUT 10%, BY WEIGHT, OF OIL-SOLUBLE, ALKALINE EARTH METAL SULFONATE AND AN ALKALINE EARTH METAL GLYCOXIDE, SAID GLYCOXIDE BEING SUFFICIENT IN AMOUNT TO INHIBIT CORROSION OF SUBSTANTIALLY PURE SILVER BY SAID COMPOSITION, SAID ZINC SALT BEING THE ONLY ESSENTIAL SULFUR-CONTAINING OXIDATION INHIBITOR PRESENT IN SAID COMPOSITION. 